Fractionation is a well developed unit operation used in the petrochemical, chemical and petroleum refining industries to separate volatile chemical compounds. In conventional fractionation columns, the feed stream is normally divided into two product streams: an overhead product (i.e. a light product) and a bottoms product (i.e. a heavy product). But, it is sometimes desirable to separate a feedstream containing a plurality of components or constituents into three component portions or fractions. When the component fractions have sufficiently different boiling points, this can be readily accomplished as is known in the art by the use of two or more fractionation columns connected in series. However, the use of a plurality of columns necessitates the use of a corresponding number of reboilers, condensers and control equipments plus the piping interconnecting the two or more columns in series. A separation in a series of fractionation columns thus entails not only considerable expenditure on plants and equipments but also a considerable energy input. The operating costs of such a multi-stage fractionation process are correspondingly high.
To save the cost for separating a multi-component stream, it has been proposed in the art to use a single column which has a divided wall disposed generally vertically within the column to effect the separation of three constituent fractions. Such an apparatus is generally referred to as a divided wall column (DWC). The divided wall or Petlyuk configuration for fractionation columns was initially introduced some 60 years ago by Petlyuk et al. DWCs have been employed for the separation of hydrocarbon mixtures as evidenced by the disclosure of U.S. Pat. No. 2,471,134 issued to R. O. Wright. Recently the use of DWCs has begun to expand because of the greater recognition that in certain situations DWCs can provide benefits above those of conventional fractionation columns.
Up to now, a number of DWCs have been developed for various applications. Depending on the position of the divided wall within a divided wall column along its longitudinal axis, existing DWCs can be divided generally into three categories.
In a first type of DWCs, the divided wall is located in a rectifying section, i.e. an upper part, of the column, partitioning the rectifying section into two separated zones, so that at least two overhead (top) products and one bottoms product can be obtained using a single column. An example of such type of DWCs can be found in U.S. Pat. No. 5,755,933 which is incorporated herein by reference.
In a second type of DWCs, the divided wall is located in a middle part of the column, dividing the inner space of the column into four zones: a stripping zone below the divided wall, a rectifying zone above the divided wall, a pre-fractionation zone at one side of the divided wall, and a main fractionation zone at the opposite side of the divided wall. A multi-component feedstream is fed into the pre-fractionation zone, an overhead (top) product is drawn off from the rectifying zone, a bottoms product is drawn from the stripping zone, and an intermediate (middle) product is drawn off from the main fractionation zone.
Examples of such type of DWCs can be found in U.S. Pat. Nos. 2,471,134, 4,230,533, 6,551,465, 7,649,108, and 7,521,576, and U.S. Published Patent Application No. 2003/0230476, each of which is incorporated herein by reference.
In a third type of DWCs, the divided wall is located in a stripping section, i.e. a lower part, of the column, partitioning the stripping section into two separated zones, so that at least one overhead (top) product and two bottoms products can be obtained using a single column. Examples of such type of DWCs can be found in U.S. Pat. No. 5,755,933 and Chinese Pat. No. 203494195U, each of which is incorporated herein by reference.
In each type of DWCs, mass transfer elements, such as trays and packings, are generally provided in one or more zones to accomplish the separation process. In addition, to support the mass transfer elements or to further enhance the separation performances, some column internals, such as liquid collectors, ring channels, gas/liquid distributors, and support members, are provided, especially in the section where the divided wall is located (i.e. divided wall section). Such column internals are generally mounted by fixing at one end to the column wall and at the other end to the divided wall, so as to achieve a secure fixation. However, such a configuration of column internals may cause some problems. For example, due to a manufacturing defect of the divided wall and/or a deformation thereof under high temperatures and/or stress during operation, a mis-alignment of column internals fixed to the divided wall may occur. Such a mis-alignment of column internals, such as gas/liquid distributors, may cause maldistribution of vapor or liquid materials over mass transfer elements like fractionation trays and/or packing, thereby deteriorating the overall separation performance of the column.
Therefore, there remains a need for a novel divided wall column, in which the above defect present in the prior art can be avoided.